Cancer cells are characterized by unlimited proliferation. If means for selectively inhibiting proliferation of cancer cells without impairing the proliferation ability of normal cells can be found, termination of tumor growth is enabled irrespective of extent of differentiation or penetration of the tumor, or degree of metastasis. Development of anticancer agents targeting for proliferating cells has been extensively carried out to provide a therapeutic drug of cancer. However, such anticancer agents have involved significant problems of accompanying cell toxicity because they also inhibit the proliferation of normal cells.
In an attempt to solve such problems, it is interesting to comprehend as to how division of cancer cells differs from that of normal cells.
Cell proliferation according to a cell cycle is determined by a holoenzyme formed by a combination of a protein called cyclin, the concentration of which varies through the cell cycle, and cyclin dependent kinase (hereinafter, referred to as “CDK”) which is converted into its active form upon binding. Further, the holoenzyme which is a cyclin/CDK complex is inhibited by a protein called cyclin dependent kinase inhibitor (hereinafter, referred to as “CDKI”) including a protein p21 WAF1/CIP1 (p21).
Various studies have been conventionally made on proteins of cyclin E gene, D gene and B1 gene, and CDK that corresponds to each cyclin dependent kinase. Cyclin/CDK2 complex and cyclin D/CDK4 or cyclin D/CDK6 complex control cell cycle progress through cell cycle checkpoint during the G1 phase and the S phase that is a DNA synthesis phase. Also, cyclin B1/CDK1 complex controls cell cycle checkpoint immediately before mitosis.
An invention relating to a diagnostic method and therapeutic method of cancer focusing attention to the aforementioned cyclin/CDK complex has been already disclosed in Japanese Translation Provisional Publication Nos. 2002-504683 and 2002-519681, and a method of measuring a cell cycle regulation factor without using a radioactive isotope (hereinafter, referred to as “RI”) has been also disclosed (Japanese Patent Provisional Publication No. 2002-335997).
When the state of G1 phase in a cell cycle is disrupted, abnormal cell proliferation is induced. A p53 molecule (tumor suppressing molecule) has been known as a protein playing a central role in mainly controlling the G1 phase of a cell cycle. It has been already reported that mutation in the p53 gene is caused at a high incidence rate in a variety of cancer tissues. A normal p53 molecule functions as a transcription factor, and controls cell proliferation, DNA repair, differentiation, and apoptosis. Moreover, in noncancerous cells having a damaged DNA, the p53 molecule promotes expression of p21 cip1/kip1 that is a CDKI molecule, suppresses endogenously existing CDK2 kinase activity, and arrests the cell cycle at the restriction (R) point on the boundary of G1/S phase. On the other hand, mutated p53 molecules can not regulate the cell cycle, but lead to proliferation of uncontrollable cells and instability of a genome, resulting in the cell turned into a malignant tumor.
A series of molecular pathological investigations have elucidated clinical significance of a CDK controlling molecule in the G1 phase. Over expression of cyclin E corresponding to CDK2 that is a cell cycle regulatory protein and cyclin D1 corresponding to CDK4 and CDK6, and inactivation of CDKI molecules, e.g., p21 corresponding to CDK2, p27 corresponding to CDK2, 4 and 6 and p16 corresponding to CDK2 are playing important roles in tumor formation in various tissues.
However, on the grounds of unestablished immunohistological techniques depending on specificity for each antibody and the like, ratios of expression of these cyclin E, cyclin D1 and the like in cancer tissues greatly vary with respect to each report.